1. Field of the Invention
The present invention relates to a technique for optically coupling an optical waveguide and an optical component, and particularly to an optical waveguide coupler, a sub-assembled optical unit, an optical module and an optically coupling method for optically coupling an optical waveguide and an optical component having mutually different spot sizes.
2. Description of the Related Art
More and more expectations have been placed on optical subscriber systems since FTTH (Fiber to the home) has become widespread in earnest. In particular, commercialization of GE-PON (Gigabit Ethernet®-Passive Optical Network) systems has been in rapid progress. In the GE-PON system, ONU (Optical Network Unit) and OLT (Optical Line Terminal) are used for sending/receiving optical signals. To these components, optical modules enabling bidirectional communication over a single cable of upstream signal 1.31 μM/downstream signal 1.49 μm are applied. These optical modules include “BiDi (Bi-Directional) modules” and “PLC (Planner Lightwave Circuit) modules. The “BiDi module” is prepared by combining micro-optics such as a LD (Laser Diode)/PD (Photo Diode), a filter and a lens. The “PLC module” is prepared by forming a silica waveguide on a silicon substrate and mounting the LD/PD and the like on the surface. For the former, an optical loss is low because of optical coupling by a lens, but active adjustment (alignment with the LD kept emitting light) is required. Therefore, production costs increase, and the production lead time gets longer. For the latter, passive alignment (alignment with the LD extinguished) by alignment using a silicon V groove and a marker is possible. Therefore, production costs are reduced, and the production lead time can be shortened. However, there is a problem that the PLC module tends to increase optical loss because of optical coupling via an optical waveguide.
Optical modules for ONU in the GE-PON system are required to be supplied in a constant number in the order of tens of thousands per month. Therefore, more and more expectations have been placed on the PLC module enabling production at a low cost and in a short time period. However, it has hitherto been difficult for the PLC module to have characteristics equivalent to those of the BiDi module as an optical module applied to the Ge-PON.
Thus, for improving the characteristics of the PLC module, a technique for optically coupling a LD and an optical waveguide, and an optical fiber and an optical waveguide with high efficiency is required. In particular, a coupling loss between the LD and the optical waveguide is very high, i.e. about several dB (e.g., 5 to 7 dB). This is a main factor which limits the characteristics. This is due to a large difference in spot size between the LD and the optical waveguide (spot size of LD<spot size of the optical waveguide). Accordingly, for reducing the coupling loss between the LD and the optical waveguide, it is necessary to enlarge the spot size of the LD or to reduce the spot size of the optical waveguide.
A SSC (Spot Size Converter) is known as a function for changing the spot size. For example, there is a LD with a SSC for increasing the spot size of the LD. However, this is expensive, and therefore unsuitable for application to an optical module intended for an optical subscriber system. It is therefore considered that reducing the spot size of the optical waveguide is a practical solution. For reducing the spot size of the optical waveguide, the core size of the optical waveguide may be reduced. However, if the core size decreases to a certain size or less, the spot size is enlarged. This results from a phenomenon in which light does not stay in the core but exudes to the clad side. If the core size is further reduced, light radiates and is no longer propagated in the core. Because there is such a limitation, a sufficient improvement of the characteristics cannot be achieved merely by reducing the core size. Thus, increasing a relative refractive index difference (Δn) of the optical waveguide so that light stays in the core has been under consideration (hereinafter such an optical waveguide is referred to as High Δ optical waveguide).
In a transceiver for bidirectional communication over a single cable, a port coupling the LD and the optical waveguide (LD port) and a port coupling the optical fiber and the optical waveguide (COM port) exist on the same flat surface of the same wafer. It is generally difficult to partially change a refractive index on the wafer flat surface. Thus, if Δn is increased for improving the coupling efficiency of the LD port, then Δn of the optical waveguide of the COM port also increases. In other words, if the spot size of the optical waveguide in the LD port is reduced, then the spot size of the optical waveguide in the COM port also decreases. Consequently, there is a problem that the coupling efficiency of the COM port decreases. Thus, the improvement of coupling efficiency in the LD port and the COM port is under a relationship of tradeoff.
Thus, a technique of performing trimming separately (e.g. applying UV exclusively to near the LD port) for partially increasing exclusively the refractive index of the LD port has been proposed. However, this results in a reduction in yield, and has a problem in terms of both the cost and production lead time.
For solving this problem, an optical coupling structure in which a High Δ optical waveguide and the optical fiber are coupled with high efficiency has been under consideration. For example, the “optical coupler” disclosed in Japanese Patent Laid-Open No. 2003-43279 (patent family US 2004/0264863) has a tapered waveguide (tapered SSC) for reducing a coupling loss between the optical waveguide and the optical fiber (SMF: Single mode fiber). This tapered waveguide has a waveguide width (core width) which decreases as the front end is approached from a predetermined starting point. This shape is intended for enlarging the spot size at a coupling part so that a radiation loss resulting from the mismatch of the spot size does not occur.
Specifically, such a taper (down taper) uses a phenomenon in which light is not fully confined to the inside of the waveguide, but instead, propagated while exuding to the periphery of the waveguide by reducing the waveguide width (core width) to a certain value or less. The down taper using exudation of light has a higher capability of enlarging the spot size than an up taper with an increased waveguide width. Further, structures using an exponential taper instead of a linear taper have been proposed. In this case, the spot size can be enlarged with a short propagation distance.
However, for enlarging the spot size by such a down taper, it is necessary to reduce the waveguide width to, for example, 1 μm or less. An additional problem is that characteristics are considerably changed with variations in the width by only about 10%. Moreover, another problem is that, because this structure uses confinement of light, its effect considerably varies depending on the wavelength. In the optical subscriber system, optical signals with wavelengths of 1.31 μm and 1.49 μm, or 1.55 μm in some cases, are used. In the transceiver for bidirectional communication over a single cable, a plurality of optical signals with these wavelengths pass through the same COM port, and therefore, the coupling loss with the optical fiber should be reduced at all of these wavelengths. Confinement of light is stronger on the shorter wavelength side (light of a shorter wavelength is more easily confined). Therefore, for allowing light of a short wavelength to exude, it is necessary to further reduce the waveguide width. However, once exudation of light begins, the spot size is abruptly enlarged. Therefore, if the waveguide width is reduced so that light of 1.31 μm exudes, light of 1.49 μm (or 1.55 μm) can no longer be coupled to the optical waveguide, and radiates to outside.
Thus, another proposal is, for example, the “waveguide taper” disclosed in Japanese Patent Laid-Open No. 8-262244 (patent family U.S. Pat. No. 5,629,999). This taper has a structure in which the optical waveguide is segmented along the direction of propagation of light. This periodically segmented waveguide (segmented SSC) uses a diffraction effect of light. In a gap at which the optical waveguide is segmented, light diffracts and the spot size is enlarged both longitudinally and laterally. Light tends to be coupled at the next optical waveguide and to maintain a plane wave (wave surface is prevented from becoming spherical).
The segmented SSC can be fabricated more easily than the tapered SSC, and its tolerance is not as strict. Therefore, the segmented SSC functions as an effective SSC in the High Δ optical waveguide by cleverly selecting a segmented period and duty-cycle (duty-cycle is a proportion of the waveguide length to the period). However, because a radiation loss occurs, the segmented SSC is inferior in coupling characteristic to the tapered SSC. In addition, it uses a diffraction effect of light, and therefore has a wavelength characteristic similar to that of the tapered SSC. The segmented SSC has a problem that decreases an effect as a SSC because the diffraction effect becomes lower as the wavelength becomes larger.